Easiest:
,
,
.
I find these champs to be the easiest to learn for they all carry simple things that each player should learn in itself. Blitz allows one to learn the importance of picks and engage in the simplest manner, you get the enemy Ash you win you get the enemy Malphite you most likely lose. Try teaches early trades and how one can both snowball a lead and how to try and fight it. Heim teaches cs and the difference range and numbers can make in a fight while also showing to prioritize the DMG of a team and not the tank.
Hardest:
,
,
.
Now this one was a challenge since no Champ is really difficult, some just need more time to get to know than others but of all the champs these are the ones I find are the hardest to play as, at least in the CURRENT META, not because they are weak but just that unless you are good with them they will do very little.
Cass is kite mage with no real mobility except a movement speed on her which if that misses it leads to her being easy pray that does little dmg for the E in itself does nothing alone, she needs a team to peel for her while at the same time no right positioning in itself. As for Sol he is a bit simpler than Cass given the stun on his Q but is weak if anyone gets close to him and with the mobility this game has now it ain't that hard to do so. Then we reach Fid and it seems like the world has been aginst him for the longest time now, and I don't mean to say his DMG is hard to do but when his DMG relies on lack of vision, his CS in the jungle is hard to do and farm and his squishiness I can't help but say he is the most difficult right now.